In 1998, Nima Arkani-Hamed found himself pondering one of the conundrums
of modern physics: why is gravity so much weaker than the other fundamental
forces?

Surrounded by massive objects like falling apples, orbiting moons, and
our own occasionally clumsy bodies, we don't think of gravity as weak.
Compared to electromagnetism, however-or the aptly named strong force
that binds quarks, or even the "weak" force that governs some
forms of radioactive decay-gravity is feeble. A pin on a tabletop is held
down by the gravity of the entire Earth; a toy magnet snaps it up.

An aerialist on a tight wire can travel
in only one dimension, forward and back. A flea on the wire finds
a "rolled-up" second dimension.

Particle accelerators have shown that electromagnetism and the weak force
are aspects of one electroweak force; they will soon attain enough energy
to observe "grand unification" with the strong force. Gravity
too is unified with these forces, but because it is weaker by scores of
orders of magnitude, accelerators can never achieve the colossal energies
needed to see its unification-unless Nima Arkani-Hamed is right.

With his colleagues Savas Dimopoulos of Stanford and Gia Dvali of New
York University, Arkani-Hamed, now of UC Berkeley and Berkeley Lab's Physics
Division, proposed that gravity may function in more than three spatial
dimensions. If so, we experience only part of its effect.

In a three-dimensional world, the strength of attraction is squared when
the distance between two masses is halved (by the inverse-square law we
learned in school). In four dimensions, however, strength varies as the
cube, in five dimensions as the fourth power, and so on. Maybe gravity
isn't weak at all; maybe it only seems that way.

How big would extra dimensions have to be? For gravity to equal the other
forces at a hundred-thousandth of a trillionth of an inch (the electroweak
scale), one extra dimension would have to be as big as the distance between
the Earth and the sun. Two extra dimensions need extend only about a millimeter,
however, and the more extra dimensions there are, the smaller they can
be.

Additional dimensions are surprisingly easy to overlook. Consider a performer
on a high wire, confined to a single dimension, forward and back. A flea
can move around the wire as well as along it, experiencing an extra dimension.

Dimensions tinier than subatomic particles, like those invoked by superstring
theory, would be impossibly hard to probe. On the other hand, with a single
extra dimension of astronomical size, gravity would have collapsed the
solar system. Between these extremes falls the millimeter scale.

Curiously, gravity has yet to be measured at much less than a millimeter.
To calculate the gravitational attraction between two masses, one of them
must be smaller than the distance separating them-easy with falling apples
and orbiting moons but impossible with, say, poppy seeds.

Moreover, "as test masses get smaller, residual electromagnetic
effects come into play and swamp gravitation," says Arkani-Hamed.
"Nobody knows what the real force of gravity is at short distances."

That situation may soon change. Clever tabletop experiments test gravity
at ever shorter distances. If they should observe a sudden startling increase
in its strength at short distances, extra dimensions are the logical suspect.

Even more dramatic effects could be produced in particle accelerators
such as the Large Hadron Collider now under construction in Europe. If
some of the particles that carry gravity escape into extra dimensions,
high-energy collisions would show a mass-energy deficit, apparently violating
the first law of thermodynamics. Conversely, regions of immensely strong
gravity might be created at short range-miniature black holes that quickly
evaporate, releasing a shower of radiation "from nowhere."

Indeed, if our three-dimensional universe resides on a "wall"
in a larger "bulk" of multiple dimensions, our world might be
just one of many worlds, or it might be folded upon itself many times.
Distant reaches of the cosmos and whole other universes might lie less
than a millimeter away.

"In this view, 'dark matter' might be just ordinary matter,"
Arkani-Hamed suggests, "because the light from a star on a fold only
one millimeter away might have to travel billions of light years 'along
the wall' to reach us. Although we feel its gravity, we haven't seen it
yet."
A wide range of other puzzles might be solved if extra dimensions are
real. And if we do the experiments, Arkani-Hamed says, "we have a
good chance of seeing evidence for or against these ideas in the next
ten years."

As recently as 1996 one science-writing pessimist was predicting the
"end of science," asserting that what isn't already known never
can be; his assumption has proved spectacularly wrong. Arkani-Hamed is
one of several young theorists who suspect there may be more dimensions
of space than meet the eye. By offering to solve some of physics' most
intractable problems, their fresh ideas have revitalized science.

Crystalline Heavens

Engineer Gerald Przybylski of the Physics Division is a Berkeley Lab
researcher who has made numerous trips to the South Pole to help build
the AMANDA array, a neutrino "telescope" consisting of strings
of detectors suspended deep in the Antarctic ice. One balmy polar day
last summer (winter in the Northern Hemisphere), Przybylski photographed
his German colleague Ralf Wischnewski against a spectacular display of
celestial art.

Wischnewski's head, conveniently blocking the sun, is ringed by a halo
and flanked by sun dogs, where the halo is crossed by a parhelic circle.
A sun pillar climbs straight up to an upside-down rainbow (upper tangent
arc) crowning the whole. Sun dogs, so-called for their tails streaming
away from the sun, are separated from it by 22 degrees, the radius of
the circular halo to which the eponymous tangent arc is tangent. The parhelic
circle passes through the center of the sun and sometimes circles the
entire sky at the sun's elevation. These effects and others appear when
the sun is low and the atmosphere is filled with drifting ice crystals.

Ice crystals are hexagonal shapes that reflect and refract light like
countless tiny prisms; long-wavelength red light is bent less than short-wavelength
blue light, so halos, sun dogs, and arcs are reddish to sunward, with
their outward edges blue.

When the sun is near the horizon, its rays refract from the inside of
crystals drifting down horizontally, creating bright sun dogs. Halos,
however, are formed mostly by reflections from the outside of randomly
oriented crystals, while both internal refraction and external reflection
contribute to pillars. The tangent arc is formed by refraction in six-sided
rods of ice shaped like pencils, not plates.

Low ice crystals are common in polar regions, and halo displays are easier
to see when the sun is near the horizon, but you don't have to travel
to the far north or the far south to see these lovely effects; any conveniently
oriented cirrus cloud will do.